When detecting an optical signal, a typical approach is to use a photo diode. Photo diodes, as an accident of design, incorporate unwanted or parasitic capacitance. The parasitic capacitance produces undesirable effects in the signal detection by lowering the signal amplitude output as a function of increasing frequency. Further, parasitic capacitance is typically found in the signal amplifying stages normally associated with the signal detector. Thus, it is desirable to minimize or eliminate the effects of this parasitic capacitance.
Typical prior art approaches to solving this problem are shown in articles, such as in an article entitled, "Achievement of Nearly Shot Noise Limited Operation in a Heterodyne Type PCM-ASK Optical Communication System", by Kikuchi, et al., at the 8th European Conference on Optical Communications, Sept. 21-24 in 1982, at Cannes, France, where the approach was to add a resistor and an inductor into the circuit. The effect of the inductor is to convert the low-pass nature of the original circuit into a bandpass response at the desired frequency. Such a bandpass response is undesirable in many existing detection circuits other than heterodyne. The resistor in this heterodyne detection approach, broadens the response and accordingly lowers the signal amplitude. Accordingly, more amplification is needed in the later stages before the signal acquires a useful level. The amplification typically involves amplification of noise signals and in the process, typically introduces new noise signals.
A further similar approach was shown in "Electronics Letters", 30 January 1986, on p. 164. In this article, no circuitry was shown, but in view of the discussion that the signal was later bandpass filtered, leads one to believe that the approach is similar to that discussed in the previous 1982 article.
The present invention follows the general teaching of prior art knowledge that it is desirable in attempting to eliminate white noise from a signal of the type comprising a single burst of high frequency signal where the burst has a waveform approximating that of a rectangular wave to pass the signal through a circuit having a matched filter response.
The present invention comprises the realization that such a matched filter response can be obtained by including only an inductor in the signal source circuit, and not a resistor, to produce a high-Q circuit whose three dB bandwidth is much narrower than the bandwidth of signal to be passed or detected. Then, after amplification of the signal to a desirable amplitude, the signal is again filtered in a two path network, or any other filter which exhibits suitable characteristics incorporating at least one transmission zero at the center frequency of the high-Q filter circuit previously used. With appropriate adjustments, this zero of the second filter cancels the pole of the first filter and, in accordance with matched filter theory, a high signal-to-noise ratio is achieved at the output of the second filter. The resulting signal minimizes noise from the signal as initially received.
It is thus an object of the present invention to provide an improved form of signal processing of signals in a high impedance circuit environment having parasitic capacitance and where the signal being processed has relatively large amounts of white noise and thus a low signal-to-noise ratio.